The present invention relates to a loudspeaker arrangement comprising
- a loudspeaker
- an input for receiving a signal
- means for boosting the amplitude of the signal received
- an amplitude detector for detecting amplitude changings in a signal part within a predetermined frequency range
- filtering means to selectively control the amplitude boost in dependence on an output signal of the amplitude detector so as to prevent overloading at strong levels of the boosted signal.
It is a commonly used technique to improve the bass reproduction in ordinary loudspeakers by providing extra amplification for the low frequencies of the signal supplied to the loudspeaker. The frequency characteristics of the loudspeakers drop within the bass frequency range. Through appropriate amplitude increase within this range, the loudspeaker diaphragm is forced to work with increased "pumping effect" within the range. This will straighten out some of the frequency characteristics. Even a comparatively small loudspeaker may then achieve a surprisingly good bass reproduction.
However, this technique has its limits, i.e., the loudspeaker ability to react correctly to rather strong intermediate signals, as the diaphragm has a limited operation stroke. If this is used to its maximum at a certain signal strength, overload will be the result if the signal becomes even stronger in periods. The same may apply to the matching amplifier and to the amplifier in question, which has been ideally adapted to the loudspeaker. It will have no purpose to let the amplifier handle signals, which the loudspeaker cannot reproduce anyway. It should therefore be accepted that such a loudspeaker system can only be operated to a certain limit, if a reasonably correct reproduction is required also of the strong signal passages.
However, it is already known that this may be subdued by detecting the signal level and adjusting the amplitude to gradually damp the increase as the signal becomes stronger. This may give a normally acceptable reproduction of too strong signals within a noticeable frequency range in falling continuation of the range in which the loudspeaker can reproduce correctly, as the signals in question will only be compressed progressively with the volume. In general a certain loudspeaker can work with increased volume without considerable overload in case of extra strong passages, as these will only be relatively attenuated.
The loudspeaker should be able to work with signal frequencies also above the range in which it is of current interest to carry out the amplitude increase mentioned. In this upper range, however, the loudspeaker can reproduce even the strong passages, which should not be attenuated by the influence of the amplitude detector. U.S. Pat. No. 4,327,250 discloses a solution for this problem. This is done by inserting a band pass or low pass filter in front of the detector unit, so that this does not react to strong amplitudes within the "undangerous" frequency range.
This is a simple solution, but with noticeable problems. When using the filter in question a certain time distortion of the signal arises, which delays the amplitude detection and its effect. The amplifier/loudspeaker unit may thus be overloaded for a short period before the detector reduces the signal level, which may be very audible. On the other hand the detector circuit may receive delayed information that a reduction has taken place within the level in question, which may result in a reduction to a level lower than wanted. In order to prevent this the adjustment speed can be reduced. However, this means that the risk of overload at the beginning of a strong passage increases due to the slowness within the adjustment system.